engleski

Changes in macrophyte composition in channels of the Bosut river basin (Eastern Croatia) between 1980-2006

In this research we have compared species composition in channels using 55 published releves originating in the period 1980-1984, as well as recently conducted field surveys at 128 localities during 2005-06. These two datasets were used as a source of information on the spatial distribution of macrophytes in determining changes in vegetation. Comparing the two datasets makes the disappearance of Hottonia palustris evident. The species was not abundant, but was frequent in the investigated associations in the 1980-84 period, and in a recent study it has entirely disappeared. The spatial distribution of Salvinia natans and Nymphaea alba in the Bosut River Basin has also considerably reduced during this 26 year period. In contrast, Ceratophyllum demersum was previously less frequent in literature data, but is considered today to be the most common and abundant species in channels. In shallower channels with developed free-floating vegetation, Lemna gibba is recorded as a newcomer. In literature data Trapa natans was associated with Nymphoides peltata. At the present, this association has become more rare as Trapa natans appears alone as a dominant species. There is a notable increase in the proportion of free-floating species in all free-floating rooted communities during the 26 year period. Originally, in the investigated area, wide rivers were the habitat of Trapa natans. However, a recent study shows the occurrence of stands of Spirodella polyrrhiza in deep and wide channels. The species often appears as a thick layer and suppresses all other species from the habitat, except Ceratophyllum demersum, which seems to prefer low light intensity and has a wide ecological tolerance. Although there have been no water quality investigations in the Bosut River Basin in the past, the decrease of the more sensitive macrophytes and the increase of the tolerant species indicate a higher eutrophication and more additional anthropogenic disturbances. There is a need for a further study of environmental changes that could be the cause of alteration in macrophyte composition in the Bosut River Basin. Changes in macrophyte composition reported here should be taken as warnings for maintenance of biological diversity in rivers and drains which harbour rare species.

changes; macrophyte composition; Bosut River Basin

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